This invention relates generally to semiconductor technology, and more particularly the invention relates to an improved method of fabricating polysilicon emitters in semiconductor devices such as silicon solar cells.
The doped polycrystalline silicon (polysilicon) contact is used in many semiconductor devices such as transistors and solar cells. The silicon solar cell, for example, comprises a silicon body having a plurality of P-doped polysilicon contacts and a plurality of N-doped polysilicon contacts for receiving electron and holes generated in the silicon body in response to impinging solar radiation.
Two desirable characteristics in a polysilicon emitter or contact are low emitter saturation current (J.sub.0) and low contact resistance. However, the two characteristics cannot both be optimized due to certain process incompatibilities. Achieving a low contact resistance requires a high temperature annealing; however, high temperature annealing out diffuses dopant from the contact into the substrate, thereby reducing the integrity of the interfacial oxide and reducing J.sub.0.
Conventionally, in fabricating the doped polysilicon contacts a doped polysilicon layer is vapor deposited on the surface of a silicon substrate. A high temperature annealing process is employed to break the interfacial oxide and activate the dopant in the polysilicon layer.
A misconception about the interfacial oxide of polysilicon emitters is that the contact resistance of polysilicon emitters is too high for device applications if the interfacial oxide is not broken completely. Consequently, the conventional process is designed to suppress any growth of the interfacial oxide before the polysilicon deposition. Normally this leads to a very thin native oxide sandwiched between the polysilicon and the substrate. The native oxide is inevitable since it forms simply by exposing the silicon wafer to air before the polysilicon deposition. Native oxide is not stable and can be broken completely at rather low temperatures. As a result, the oxide breaking processes is incorporated in the process of drive-in for the polysilicon emitter formation.
The present invention is directed to a novel two-step annealing process which yields polysilicon emitters with enhanced low contact resistance and low emitter saturation current.